Just wanted to give you a quick update to let you know that I received the funds from Kickstarter last week, and this week I finished up the redesign of the board and ordered the prototypes! They'll be ready around the first week of May.
Here's some screenshots of the new board layout, front and back:
And because it might be hard to spot the differences, here's the old design:
So you're probably thinking the new version looks really similar to the old one. Let me go over what's changed and what hasn't and why.
First the power switch. It's changed, but it's hard to tell because they use the same form factor. Originally, I was going to go with a larger switch and mount it on the front, but then it occurred to me that people would want to be able to mount the screen flush with an enclosure, and the bigger switch was taller even than the USB connector, and that just wouldn't do. I also couldn't mount it on the back because it was taller than the spacers on a pin header and would cause issues when using a breadboard. So, I ordered several similar switches from different manufacturers on Digikey and Mouser and tried them out to see if they were better made than the originals that were made by C&K. I finally settled on an ALPS switch from Mouser which could slide easier and withstood my stress tests.
Below the power switch, you can see the diode and voltage regulator. The diode replaces the old power multiplexer, and the voltage regulator remains unchanged. Next to them however, I added a a green power indicator LED. The status LED on Pin 13 remains, down by the FFC connector for the display, but is now red.
To reduce the manufacturing costs I replaced all the 10uF caps on the board with slightly larger house parts, and to save space for the larger caps in place, I reduced the size of some of the resistors to 0402. And while I was at it I swapped the crystal for a slightly larger one that was marginally cheaper. I also swapped the reset switch for the better and cheaper house part I now use on the Tau and Neutrino.
One thing I did not change was the OLED's power circuity, but it's not for lack of trying. The current part is a Fairchild FAN5331SX regulator which is $0.50 @ 100 units but I discovered Diodes makes the AP3012KTR-G1 which is half the price and it appeared it would not require the 180pF feeedback capacitor which isn't a house part, further reducing the costs. Unfortunately I found out why the AP3012 was so cheap after I spent a lot of time poring over the datasheet and trying to calculate which resistors to use for the feedback circuit to set the output voltage. With the FAN5331 I was able to use a 10K and 100K resistor to get 13.5V, but with the Diodes part the feedback voltage reference was slightly different so the 10K and 100K would not work, and none of the other house resistor values would give me an output voltage that would remain in the range allowed for the display when taking into account the 5% tolerance. This in turn prompted me to examine the graphs of the feedback voltage reference by temperature to see how much that was going to screw with the output voltage and I saw that the Diodes part had a very significant change over temperature which would make it very difficult if not impossible to keep the output voltage in the 1V window allowed for the display, The Fairchild part's voltage reference on the other hand was very stable over different temperatures, so it was much less of a concern. Of course, running the display a bit on the high side outside of it's parameters would probably have been okay since it does have an absolute max of 16V input and the recommended max is 13.5V but I didn't want to stress the display if I didn't need to, and if I needed custom 1% resistor values to get the output voltage in the right range that was going to kill any savings I would have gotten by switching to the new part anyway. So in the end I decided to stick to the Fairchild part and chalked it up as a learning exercise.
A couple other minor changes I made was to move the MicroSD card over a bit and to switch to the house part USB connector I've been using on my latest boards.
By the way if the term "house part" confuses you, house parts are parts Macrofab keeps in stock and doesn't charge placement fees for because they have the parts already programmed into their pick and place machines. The placement fees can double the cost of a part, so using as many house parts as possible is a cost savings.
Speaking of which, one last part I changed was the 10uF inductor o the OLED voltage regulator. Macrofab had a house part for that which was slightly larger than the one I used, so I went with that instead.
The final thing to note is the change of the OUT pin to GND which I've mentioned before. Without the power multiplexing circuity there any more, the USB pin serves the same purpose, providing VIN or USB voltage whichever is greater. I could have made OUT just connect to the USB pin, but I think the extra GND is more convenient for when you have an I2C part you want to connect to the board, and it keeps it consistent with all my other designs.
So, I think that about covers it. Next update will probably be when the prototypes arrive, which as I mentioned above will be around the first week of May!